Conductive compositions and the use thereof

ABSTRACT

Use of a composition comprising finely divided particles of (a) an electrically-conductive material; (b) one or more inorganic binders; and (c) one or more metal(s) selected from cobalt, nickel, iron and bismuth, wherein components (a), (b) and (c) are dispersed in a liquid vehicle, in the manufacture of an electrically-conductive pattern on a substrate for the purpose of increasing the resistivity of said electrically-conductive pattern.

This application is a divisional application of Ser. No. 11/522,575filed on Sep. 18, 2006. Ser. No. 11/522,575 is a divisional of Ser. No.10/474,355 filed on Oct. 7, 2003. Ser. No. 10/474,355 is a NationalStage entry of PCT/US02/10504 filed Apr. 4, 2002, which claims priorityto GB 0108888.9 filed Sep. 4, 2001.

The present invention relates to conductor compositions which includeone or more metals selected from cobalt, nickel, iron, and bismuth andthe use of the compositions in the manufacture of components,particularly heating elements, in microelectronic circuits. Thesecompositions are of particular use in the manufacture of demistingelements in heated windows, for example in automotive glazing,particularly automotive backlights.

BACKGROUND OF THE INVENTION

The use of thick-film conductors as components in hybrid microelectroniccircuits is well known in the electronics field. Compositions for themanufacture of such components usually take the form of a paste-likesolid-liquid dispersion, where the solid phase comprises finely dividedparticles of a noble metal or a noble metal alloy or mixtures thereofand an inorganic binder. The liquid vehicle for the dispersion istypically an organic liquid medium, but may also be an aqueous-basedliquid medium. Additional materials may be added in small quantities(generally less than about 3% by weight of the composition) to modifythe properties of the composition and these include staining agents,rheology modifiers, adhesion enhancers and sintering modifiers.

The metals used in the preparation of thick-film conductor compositionsare typically selected from silver, gold, platinium and palladium. Themetal can be used either in isolation or as a mixture which forms analloy upon firing. Common metal mixtures include platinum/gold,palladium/silver, platinum/silver, platinum/palladium/gold andplatinum/palladium/silver. The most common systems used in themanufacture of heating elements are silver and silver/palladium. Theinorganic binder is typically a glass or glass-forming material, such asa lead silicate, and functions as a binder both within the compositionand between the composition and substrate onto which the composition iscoated. Due to environmental considerations, the use of lead-containingbinders is becoming less common and lead-free binders such as zinc orbismuth borosilicates are now often employed. The role of the organicmedium is to disperse the particulate components and to facilitate thetransfer of the composition onto the substrate.

The consistency and rheology of the composition is adjusted to theparticular method of application which may comprise screen printing,brushing, dipping, extrusion, spraying and the like. Typically, screenprinting is used to apply the composition. The pastes are usuallyapplied to an inert substrate, such as an alumina, glass, ceramic,enamel, enamel-coated glass or metal substrate, to form a patternedlayer. The thick-film conductor layer is normally dried and then fired,usually at temperatures between about 600 and 900° C., to volatilise orburn off the liquid vehicle and sinter or melt the inorganic binder andthe metal components. Direct wet-firing, i.e. wherein the thick filmlayer is not dried before firing, has also been used to generate thepatterned layer.

It is, of course, necessary to connect the conductive pattern to theother components of the electronic circuit, such as the power source,resistor and capacitor networks, resistors, trim potentiometers, chipresistors and chip carriers. This is generally achieved by using metalclips, typically comprising copper, which are soldered either directlyadjacent to or on top of the conductive layer. Where the clips aresoldered on top of the conductive layer, attachment is either directlyonto the conductive pattern itself or onto a solderable compositionwhich is overprinted onto the pattern (an “over-print”). An over-printis generally applied only in the region of the conductive pattern towhich the metal clips are attached by solder, which region is generallyreferred to as the “clip area”. The ability to solder onto theelectrically-conductive layer is an important parameter in themanufacture of heating elements since it removes the requirement for anover-print. However, the inorganic binder, which is important forbinding the paste onto the substrate, can interfere with solder wettingand result in poor adhesion of the soldered metal clips to theconductive layer. The requirements of high substrate adhesion and highsolderability (or adhesion of the metal clips to the conductive pattern)are often difficult to meet simultaneously. U.S. Pat. No. 5,518,663provides one solution to this problem by incorporating into thecomposition a crystalline material from the feldspar family.

An important application of patterned electrically-conductive layers isin the automobile industry, and particularly in the manufacture ofwindows which can be defrosted and/or demisted by anelectrically-conductive grid permanently attached to the window andcapable of producing heat when powered by a voltage source. In order forthe window to defrost quickly, the circuit must be capable of supplyinglarge amounts of power from a low voltage power source, typically 12volts. For such power sources the resistivity requirement of theconductive pattern is generally in the range of from about 2 to about 5μ·cm (5 m·/Y at 10 μm after firing). This requirement is readily met byconductors containing noble metals, particularly silver which is themost commonly-used material for this application.

In certain applications, a conductive composition having a higherresistivity is required. In particular, it is anticipated that theresistance requirements of window-heating elements in automobiles willshortly need to change since the automotive industry is expected toadopt the use of a 42 and 48 volt power supply in the near future. As aresult, the conductive composition used to manufacture thewindow-heating elements will be required to exhibit higher values ofresistivity, typically greater than about 10 μ·cm, preferably greaterthan about 12 μ·cm, particularly in the range from about 20 to about 70μ·cm.

A number of different materials may be added to adjust the specificresistivity of a conductive composition. For example, metal resinatessuch as rhodium and manganese resinates have been used to increaseresistivity, as disclosed in U.S. Pat. No. 5,162,062 and U.S. Pat. No.5,378,408. In addition, an increase in the content of precious metals,particularly the platinum group metals such as platinum and palladium,has also been used to increase the specific resistivity.Silver/palladium and silver/platinum compositions can achieveresistivity values from about 2 μ·cm (that of a composition comprisingonly silver and binder) up to around 100 μ·cm (for a 70:30 Pd:Ag blend).Systems comprising platinum and/or palladium are, however, significantlymore expensive and their use would be prohibitive in applicationsrequiring coverage of a large surface area, such as the window-heatingelements used in the automotive industry. In addition, an over-print ofa composition containing a high amount of silver (and typically smallamounts of filler) is generally required for certain metal blends, suchas compositions containing high palladium levels, in order to achieveadequate solder adhesion. Conventional conductive compositions whichtypically operate at resistivity values of 2 to 5 μ·cm and which arecomprised predominantly of silver do not require an over-print sinceacceptable levels of solder adhesion can be achieved by adjusting thelevels of inorganic binder.

Other, lower-cost approaches for achieving a high resistivity involveblending large amounts of filler into a silver-containing conductivecomposition to block the conductive path. Fillers are typicallyinorganic materials and those commonly used are glass (which may be thesame or different as that used for the binder) and alumina (or othermetal oxides). However, such approaches tend to result in a loss ofsolder acceptance and solder adhesion. For example, adequate solderadhesion can be maintained only up to a level of about 10% alumina byweight of the composition but this level is generally too low for anappreciable rise in resisitivity. For glass-type fillers, loss of solderadhesion occurs at even lower levels and, again, this level is too lowfor an appreciable rise in resistivity. In addition, this problem cannot normally be ameliorated by the use of silver over-prints owing toglass migration between the layers during firing, specifically from theconductive coating into the over-print.

A further advantageous property of the conductor compositions ischemical durability and resilience to exposure to varying environmentalconditions such as temperature, humidity, acid and salt. Compositionscomprising large amounts of glass filler, particularly lead-free glassfiller, are often relatively unstable to such factors.

An additional consideration is that it is desirable for the resistanceof the coating composition to be substantially independent of thetemperature of firing used in the manufacture of the patternedconductive layer. For instance, in the case of the application of aconductive composition to a glass substrate, the behaviour of thecomposition under sintering and melting should remain substantiallyconstant between the temperatures of about 620 and 680° C. Nevertheless,a change in resistance of up to about 10% between these twotemperatures, which corresponds to the behaviour of a pure silvercomposition, is generally tolerated. The use of large amounts of fillerto significantly increase resistivity results in compositions which donot generally satisfy this requirement.

A further additional consideration is that it is desirable for therelationship between the resistivity and the amount of resistivitymodifier added to the composition to be relatively predictable and/orsubstantially linear within the target range of desired resistivities.The resistivity of compositions comprising large amounts of fillergenerally increases in an almost linear manner until a criticalconcentration is reached. At this critical concentration, theresistivity may rise very rapidly, often by an order of magnitude, whenthe level of resistivity modifier is increased by only a fraction of aweight percent. As a result, it is difficult to target specific valuesof resistivity for such compositions.

It is an object of this invention to provide higher-resistivityelectrically-conductive compositions which do not suffer from theaforementioned disadvantages. In particular, it is an object of thisinvention to provide an economical electrically-conductive coatingcomposition having increased resistivity while at the same timeexhibiting good solderability.

SUMMARY OF THE INVENTION

The invention is directed to the use of finely-divided particles of oneor more metal(s) selected from cobalt, nickel, iron and bismuth in acomposition further comprising finely divided particles of (a) anelectrically-conductive material and (b) one or more inorganic bindersdispersed in a liquid vehicle, for the purpose of increasing theresistivity of an electrically-conductive pattern manufactured from saidcomposition.

According to a further aspect of the invention, there is provided theuse of a composition comprising finely divided particles of (a) anelectrically-conductive material; (b) one or more inorganic binders; and(c) one or more metal(s) selected from cobalt, nickel, iron and bismuth,wherein components (a), (b) and (c) are dispersed in a liquid vehicle,preferably an organic medium, for the purpose of increasing theresistivity of an electrically-conductive pattern on a substrate.

According to a further aspect of the invention, there is provided amethod for increasing the resistivity of an electrically-conductivepattern manufactured from a composition comprising finely-dividedparticles of (a) an electrically-conductive material and (b) one or moreinorganic binders dispersed in a liquid vehicle, said method comprisingthe incorporation of finely-divided particles of (c) one or moremetal(s) selected from cobalt, nickel, iron and bismuth into saidcomposition.

According to a further aspect of the invention there is provided aprocess for the manufacture of an electrically-conductive pattern, saidprocess comprising applying to a substrate a composition comprisingfinely divided particles of (a) an electrically-conductive particles;(b) one or more inorganic binders; and (c) one or more metal(s) selectedfrom cobalt, nickel, iron and bismuth, said components (a), (b) and (c)being dispersed in a liquid vehicle, preferably an organic medium, andfiring the coated substrate to effect sintering of the finely-dividedparticles to the substrate. Preferably the process is a screen printingprocess.

According to a further aspect of the present invention there is provideda substrate, typically a rigid substrate such as a glass (includingtoughened and laminated glass), enamel, enamel-coated glass, ceramic,alumina or metal substrate, having on one or more surfaces thereof anelectrically-conductive pattern, said conductive pattern comprising (a)an electrically-conductive material; (b) one or more inorganic binders;and (c) one or more metal(s) selected from cobalt, nickel, iron andbismuth.

According to further aspects of the present invention, there is provideda novel composition, per se, as defined herein, and in particular:

(i) a composition comprising finely divided particles of (a) anelectrically-conductive material; (b) one or more inorganic binders; and(c) cobalt, wherein components (a), (b) and (c) are dispersed in aliquid vehicle, preferably an organic medium;

(ii) a composition comprising finely divided particles of (a) silver;(b) one or more inorganic binders; and (c) nickel, wherein components(a), (b) and (c) are dispersed in a liquid vehicle, preferably anorganic medium, wherein component (a) is present in amounts of about 50to about 98% by weight of the total solids present in the composition,and wherein component (c) is present in amounts of about 15 to about 45%by weight of the total solids present in the composition;

(iii) a composition comprising finely divided particles of (a) silver;(b) one or more inorganic binders; and (c) iron, wherein components (a),(b) and (c) are dispersed in a liquid vehicle, preferably an organicmedium; and

(iv) a composition comprising finely divided particles of (a) silver;(b) one or more inorganic binders; and (c) bismuth, wherein components(a), (b) and (c) are dispersed in a liquid vehicle, preferably anorganic medium, and wherein component (c) is present in amounts of about15 to about 45% by weight of the total solids present in thecomposition.

DETAILED DESCRIPTION OF THE INVENTION

In one embodiment, the composition comprises only one metal selectedfrom cobalt, nickel, iron and bismuth. In one embodiment, component (c)is selected from cobalt, nickel and iron. In a further embodiment,component (c) is selected from cobalt and nickel.

The composition preferably exhibits values of resistivity of greaterthan about 10 μ·cm, preferably greater than about 12 μ·cm, preferably inthe range from about 20 to about 70 μ·cm, and more preferably in therange from about 20 to about 50 μ·cm. Thus, as used herein, the term“increasing the resistivity” means increasing the resistivity preferablyto a value of resistivity of greater than about 10 μ·cm, preferablygreater than about 12 μ·cm, preferably in the range from about 20 toabout 70 μ·cm, and more preferably in the range from about 20 to about50 μ·cm. In one embodiment, the resistivity is in the range from about30 to about 40 μ·cm.

The compositions described herein are suitable for use as pastecompositions for forming thick-film conductive patterns on a substrate,for instance, by the process of screen-printing. The compositions of thepresent invention are of particular use as components in the manufactureof windows which can be defrosted and/or demisted by anelectrically-conductive grid attached to the window, particularly foruse in the automotive industry.

As used herein, the term “finely divided” is intended to mean that theparticles are sufficiently fine to pass through a 400-mesh screen (USstandard sieve scale). It is preferred that at least 50%, preferably atleast 90%, and more preferably substantially all of the particles are inthe size range of 0.01 to 20 μm. Preferably, the largest dimension ofsubstantially all particles is no more than about 10 μm and desirably nomore than about 5 μm.

Preferably, the components are present in amounts such that the totalamount of components (a), (b) and (c) is about 50 to about 95% by weightof the composition, with the liquid vehicle being present in amounts ofabout 5 to about 50% by weight of the composition. In a preferredembodiment, the total amount of components (a), (b) and (c) is in therange from about 60 to about 90%, preferably from about 70 to about 85%by weight of the composition.

Compounds (a), (b) and (c) generally comprise substantially all of thesolid phase material used to prepare the compositions used in theinvention.

Preferably component (a) is present in amounts of from about 30 to about99.4%, preferably from about 40 to about 98% by weight of the totalsolids present in the composition. In one embodiment, component (a) ispresent in amounts of from about 40 to about 70% by weight of the totalsolids present in the composition. In a further embodiment, component(a) is present in amounts of from about 45 to about 65% by weight of thetotal solids present in the composition.

Preferably component (b) is present in amounts of from about 0.5 toabout 40%, preferably from about 1 to about 25% by weight of the totalsolids present in the composition. In one embodiment, component (b) ispresent in amounts of from about 2 to about 15% by weight of the totalsolids present in the composition. In a further embodiment, component(b) is present in amounts of from about 2 to about 20% by weight of thetotal solids present in the composition.

Preferably component (c) is present in amounts of about 2 to about 45%by weight of the total solids in the composition. In one embodiment,component (c) is present in amounts of about 2 to about 30%, preferablyfrom about 5 to about 30%, from about 10 to about 30% by weight of thetotal solids present in the composition. In one embodiment, component(c) is present in amounts of from about 15 to about 30% by weight of thetotal solids present in the composition. In a further embodiment,component (c) is present in amounts of from about 15 to about 45%,preferably about 25 to about 45% by weight of the total solids presentin the composition.

The electrically-conductive particles of component (a) can be in anyform suitable for the production of the compositions of the presentinvention. For example, electrically-conductive metallic particles maybe in the form of either metal powders or metal flakes or blendsthereof. In one embodiment of the invention, the metallic particles area blend of powder and flake. The particle size of the metal powder orflake is not by itself narrowly critical in terms of technicaleffectiveness. However, particle size does affect the sinteringcharacteristics of the metal in that large particles sinter at a lowerrate than small particles. Blends of powders and/or flakes of differingsize and/or proportion can be used to tailor the sinteringcharacteristics of the conductor formulation during firing, as iswell-known in the art. The metal particles should, however, be of a sizethat is appropriate to the method of application thereof, which isusually screen printing. The metal particles should therefore generallybe no larger than about 20 μm in size and preferably less than about 10μm. The minimum particle size is normally about 0.1 μm.

The preferred metal for the electrically-conductive component (a) of theconductor composition is silver. Silver particles larger than about 1.0μm impart greater colouring to the composition. It is preferred that thecompositions contain at least 50% weight silver particles larger than1.0 μm. The silver will ordinarily be of high purity, typically greaterthan 99% pure. However, less pure materials can be used depending on theelectrical requirements of the conductive layer or pattern. In anembodiment of the invention, component (a) comprises a mixture of silverand nickel and/or suitable derivatives. A preferred nickel derivativesuitable for use in this embodiment of the invention is nickel boride(Ni₃B). Typically, the Ag:Ni ratio will be about 1:1 to about 25:1,preferably at least about 1.5:1 and more preferably about 1.5:1 to about3:1. It will be understood by the skilled person that reference hereinto the electrically-conductive component (a), and to the relativeamounts thereof, does not include reference to component (c) or to therelative amounts thereof, even though the particles of component (c) maythemselves be electrically-conductive. Equally, a reference to theparticles of component (c) and the relative amounts thereof does notinclude reference to the electrically-conductive particles of component(a) and the relative amounts thereof, even though the particles ofcomponent (c) may themselves be electrically-conductive.

Component (c) in the compositions used in the present inventioncomprises the metal in one or more of the following forms:

(i) metallic particles of one or more metal(s) selected from cobalt,nickel, iron and bismuth;

(ii) particles of one or more alloy(s) containing one or more metal(s)selected from cobalt, nickel, iron and bismuth;

(iii) one or more derivative(s) of one or more metal(s) selected fromcobalt, nickel, iron and bismuth, wherein the derivative issubstantially converted to the metal under the action of heat.

Preferably, the particles of component (c) are metallic particles,and/or particles of one or more alloy(s). More preferably, the particlesof component (c) are metallic particles of one or more metal(s) selectedfrom cobalt, nickel, iron and bismuth.

The size of the particles should generally be no larger than about 20 μmand preferably less than 10 μm. The minimum particle size is normallyabout 0.1 μm. The particles may be spherical or spheroid or irregular inshape, in the form of a flake or a powder, or in any other suitablemorphology.

The use of component (c) as an additive provides compositions whichexhibit (i) high resistivity; and (ii) high solder adhesion; preferablyalso (iii) a more uniform rise in resistivity with increasingconcentration of the additive in relation to compositions in which largeamounts of filler are used to increase resistivity; and preferably also(iv) low variation of resistance with firing temperature. In addition,cobalt, nickel, iron and bismuth are relatively inexpensive andrepresent an economical method of increasing resistivity.

Suitable inorganic binders for use in the present invention are thosematerials which upon sintering serve to bind the metal to a substratesuch as a glass (including toughened and laminated glass), enamel,enamel-coated glass, ceramic, alumina or metal substrate. The inorganicbinder, also known as a frit, comprises finely-divided particles and isa key component in the compositions described herein. The softeningpoint and viscosity of the frit during firing, as well as its wettingcharacteristics for the metal powder/flake and the substrate, areimportant factors. The particle size of the frit is not narrowlycritical and frits useful in the present invention will typically havean average particle size from about 0.5 to about 4.5 μm, preferably fromabout 1 to about 3 μm.

It is preferred that the inorganic binder is a frit having a softeningpoint of between about 350 and 620° C. in order that the compositionscan be fired at the desired temperatures (typically 300 to 700° C.,particularly 580 to 680° C.) to effect proper sintering, wetting andadhesion to the substrate, particularly a glass substrate. It is knownthat mixtures of high and low melting frits can be used to control thesintering characteristics of the conductive particles. In particular, itis believed that the high temperature frit dissolves in the lowermelting frit and together they slow the sintering rate of the conductiveparticles as compared to pastes containing only low melting frit. Thiscontrol of the sintering characteristics is especially advantageous whenthe composition is printed and fired over decorative enamels.(Decorative enamels are normally pastes comprised of one or more pigmentoxides and opacifiers and glass frit dispersed in an organic medium.) Ahigh melting frit is considered to be one having a softening point above500° C. and a low melting frit is considered to be one having asoftening point below 500° C. The difference in the melting temperaturesof the high and low melting frits should be at least 100° C. andpreferably at least 150° C. Mixtures of three or more frits havingdifferent melting temperatures can also be used. When mixtures of highand low melting frits are used in the invention, they are normally usedin ratios by weight from 4:1 to 1:4. As used herein, the term “softeningpoint” refers to softening temperatures obtained by the fibre elongationmethod of ASTM C338-57.

Suitable binders include lead borates, lead silicates, leadborosilicates, cadmium borate, lead cadmium borosilicates, zincborosilicates, sodium cadmium borosilicates, bismuth silicates, bismuthborosilicates, bismuth lead silicates and bismuth lead borosilicates.Typically, any glass having a high content of bismuth oxide, preferablyat least 50% and more preferably at least 70% by weight bismuth oxide,is preferred. Lead oxide as a separate phase may also be added, ifnecessary. However, due to environmental considerations, lead-freebinders are preferred. Examples of glass compositions (compositions A toI) are given in Table 1 below; the oxide components are given in weightpercent.

TABLE 1 Glass Compositions A B C D E F G H I Bi₂O₃ 75.1 82.7 78.1 94.873.3 73.7 69.82 PbO 10.9 1.83 43.6 0.7 B₂O₃ 1.2 1.34 4.8 26.7 8.38 SiO₂9.3 10.3 37.5 21.7 8.6 5.2 4.7 4.8 7.11 CaO 2.4 2.68 9.7 4.0 0.53 BaO0.9 ZnO 27.6 3.9 5.0 12.03 CuO 7.6 5.5 CoO 1.8 Al₂O₃ 1.1 1.22 4.3 5.72.13 Na₂O 8.7 ZrO₂ 4.0 GeO₂ 16.5 16.6

The glass binders are prepared by conventional glass-making techniques,by mixing the desired components (or precursors thereof, e.g., H₃BO₃ forB₂O₃) in the desired proportions and heating the mixture to form a melt.As is well known in the art, heating is conducted to a peak temperatureand for a time such that the melt becomes entirely liquid, yet gaseousevolution has ceased. The peak temperature is generally in the range1100° C.-1500° C., usually 1200° C.-1400° C. The melt is then quenchedby cooling the melt, typically by pouring onto a cold belt or into coldrunning water. Particle size reduction can then be accomplished bymilling as desired.

Other transition metal oxides may also be employed as part of theinorganic binder, as is well known to those skilled in the art. Oxidesor oxide precursors of zinc, cobalt, copper, nickel, manganese and ironare commonly used, particularly with substrates other than glasssubstrates, such as alumina substrates. These additives are known toimprove soldered adhesion.

The inorganic binder can also contain up to approximately 4 parts byweight basis paste of a pyrochlore-related oxide having the generalformula:

(M_(x)M′_(2-x))M″₂O_(7-Z)

wherein

M is selected from at least one of Pb, Bi, Cd, Cu, Ir, Ag, Y and rareearth metals having atomic numbers of 57-71 and mixtures thereof,

M′ is selected from Pb, Bi and mixtures thereof,

M″ is selected from Ru, Ir, Rh and mixtures thereof,

X=0-0.5, and

Z=0-1.

Pyrochlore materials have been described in detail in U.S. Pat. No.3,583,931, the disclosure of which is incorporated herein by reference.The pyrochlore materials act as adhesion promoters for the compositionsof this invention. Copper bismuth ruthenate(Cu_(0.5)Bi_(1.5)Ru₂O_(6.75)) is preferred.

Traditionally, conductive compositions have been based on lead frits.The elimination of lead from glass compositions to meet current toxicityand environmental regulations may limit the types of binder that can beused to achieve the desired softening and flow characteristics, whilesimultaneously meeting wettability, thermal expansion, cosmetic andperformance requirements. U.S. Pat. No. 5,378,406, the disclosure ofwhich is incorporated herein by reference, describes a series oflow-toxicity lead-free glasses based upon the constituents Bi₂O₃, Al₂O₃,SiO₂, CaO, ZnO and B₂O₃, all of which may be used in the compositionsdescribed herein. In a preferred embodiment, the frit is composition Iin Table 1 herein.

The components (a) to (c) of the composition hereinbefore described willordinarily be dispersed into a liquid vehicle to form a semi-fluid pastewhich is capable of being printed in a desired circuit pattern. Theliquid vehicle may be an organic medium or may be aqueous-based.Preferably the liquid vehicle is an organic medium. Any suitably inertliquid can be used as an organic medium. The liquid vehicle shouldprovide acceptable wettability of the solids and the substrate, arelatively stable dispersion of particles in the paste, good printingperformance, dried film strength sufficient to withstand rough handling,and good firing properties. Various organic liquids with or withoutthickening agents, stabilising agents and/or other common additives aresuitable for use in the preparation of the compositions of the presentinvention. Exemplary of the organic liquids which can be used arealcohols (including glycols); esters of such alcohols such as theacetates, propionates and phthalates, for instance dibutyl phthalate;terpenes such as pine oil, terpineol and the like; solutions of resinssuch as polymethacrylates of lower alcohols; or solutions of ethylcellulose in solvents such as pine oil and monobutyl ether of diethyleneglycol. The vehicle can also contain volatile liquids to promote fastsetting after application to the substrate.

A preferred organic medium is based on a combination of a thickenerconsisting of ethyl cellulose in terpineol (typically in a ratio of 1 to9), optionally combined for instance with dibutyl phthalate or with themonobutyl ether of diethylene glycol (sold as butyl Carbitol™). Afurther preferred organic medium is based on ethyl cellulose resin and asolvent mixture of alpha-, beta- and gamma-terpineols (typically 85-92%alpha-terpineol containing 8-15% beta and gamma-terpineol).

The ratio of liquid vehicle to solids in the dispersion can varyconsiderably and is determined by the final desired formulationviscosity which, in turn, is determined by the printing requirements ofthe system. Normally, in order to achieve good coverage, the dispersionswill contain about 50 to about 95%, preferably about 60 to about 90%, byweight solids, and about 5 to about 50%, preferably about 10 to about40%, by weight liquid vehicle, as noted above.

The compositions described herein may additionally comprise furtheradditives known in the art, such as colorants and staining agents,rheology modifiers, adhesion enhancers, sintering inhibitors,green-state modifiers, surfactants and the like.

In the preparation of the compositions described herein, the particulateinorganic solids are mixed with the liquid vehicle and dispersed withsuitable equipment, such as a three-roll mill or a power-mixer,according to conventional techniques well-known in the art, to form asuspension. The resulting composition has a viscosity generally in therange of about 10-500, preferably in the range of about 10-200, morepreferably in the range of about 15-100 Pa·s at a shear rate of 4 sec⁻¹,for instance, as measured on a Brookfield HBT viscometer using #5spindle at 10 rpm and 25° C. The general procedure for preparing thecompositions described herein is set out below.

The ingredients of the paste are weighed together in a container. Thecomponents are then vigorously mixed by a mechanical mixer to form auniform blend; then the blend is passed through dispersing equipment,such as a three-roll mill, to achieve a good dispersion of particles toproduce a paste-like composition having a suitable consistency andrheology for application onto a substrate, for instance byscreen-printing. A Hegman gauge is used to determine the state ofdispersion of the particles in the paste. This instrument consists of achannel in a block of steel that is 25 μm deep (1 mil) on one end andramps up to zero depth at the other end. A blade is used to draw downpaste along the length of the channel. Scratches appear in the channelwhere the agglomerates' diameter is greater than the channel depth. Asatisfactory dispersion will give a fourth scratch point of typically10-18 μm. The point at which half of the channel is uncovered with awell-dispersed paste is between 3 and 8 μm typically. Fourth scratchmeasurements of >20 μm and “half-channel” measurements of >10 μmindicate a poorly dispersed suspension.

The compositions are then applied to a substrate using conventionaltechniques known in the art, typically by the process of screenprinting, to a wet thickness of about 20-60 μm, preferably about 35-50μm. The compositions described herein can be printed onto the substrateseither by using an automatic printer or a hand printer in theconventional manner. Preferably, automatic screen printing techniquesare employed using a 200- to 325-mesh per inch screen. The printedpattern is optionally dried at below 200° C., preferably at about 150°C., for a time period between about 30 seconds to about 15 minutesbefore firing. Firing to effect sintering of both the inorganic binderand the finely divided particles of metal is preferably done in awell-ventilated belt conveyor furnace with a temperature profile thatwill allow burn-off of the vehicle at about 200-500° C., followed by aperiod of maximum temperature of about 500-1000° C., preferably about600-850° C., lasting for about 30 seconds to about 15 minutes. This isfollowed by a cooldown cycle, optionally a controlled cooldown cycle, toprevent over-sintering, unwanted chemical reactions at intermediatetemperatures or substrate fracture which can occur from too rapidcooldown. Alumina substrates are particularly susceptible to fractureresulting from too rapid cooldown. The overall firing procedure willpreferably extend over a period of about 2-60 minutes, with about 1-25minutes to reach the firing temperature, about 10 seconds to about 10minutes at the firing temperature and about 5 seconds to about 25minutes in cooldown. For the manufacture of a toughened glass substrate,a controlled cooldown cycle is generally used wherein the overall firingprocedure typically extends over a period of about 2 to 5 minutes, withabout 1 to 4 minutes to reach the firing temperature, followed by arapid cooldown.

Typical thicknesses of the thick-films after firing are from about 3 μmto about 40 μm, preferably from about 8 μm to about 20 μm.

The compositions described herein are primarily intended for use in themanufacture of heating elements in windows such as defogging ordefrosting elements in automotive glazing, particularly backlights. Thecompositions may also be used to incorporate other conductive functionsinto the window, such as a printed aerial or antenna. However, thecoating compositions can be employed in various other applications,including printed circuits and heating elements generally. For instance,the compositions described herein may be used as base plates in hotwater heating appliances. There is a general need within the electronicsand electrical industry for lower-cost heating elements, particularlyscreen-printable heating elements.

The following procedures were used to evaluate the compositionsdescribed herein.

Test Procedures Adhesion

Copper clips (obtained from Quality Product Gen. Eng. (Wickwar), UK) aresoldered to the fired conductive pattern on a glass substrate(dimensions 10.2 cm×5.1 cm×3 mm) using a 70/27/3 Pb/Sn/Ag solder alloyat a soldering iron temperature of 350 to 380° C. A small quantity of amildly active rosin flux, such as ALPHA 615-25® (Alpha Metals Limited,Croydon, U.K.) may be used to enhance solder wetting and to keep thesolder and clip in place during assembly of parts, in which case theflux is applied to the solder using a shallow tray containing a thinfilm of fresh flux. Adhesion was measured on a Chattillon® pull testerModel USTM at a pull speed of 0.75±0.1 inches per minute (1.91±0.25 cmper minute) and the pull strength recorded at adhesion failure. Theaverage value of adhesion failure over 8 samples was determined. Theadhesion should preferably be greater than 10 kg, more preferablygreater than 15 kg and more preferably greater than 20 kg. The principalfailure modes of adhesion are as follows:

(a) clip separates from the conductive pattern (i.e. poor solderadhesion).

(b) the conductive pattern separates from the substrate (i.e. poorsubstrate adhesion).

(c) glass pullout/fracture (i.e. the bonding strengths between the clipand the conductive layer and between the conductive layer and thesubstrate is greater than the strength of the substrate.

(d) failure within the solder.

Resistance and Resistivity

The resistance of the fired conductive pattern on a glass substrate(dimensions 10.2 cm×5.1 cm×3 mm) was measured using a GenRad Model 1657RLC bridge calibrated for use between 1 and 900 • or equivalent. Thethickness of the conductive layer is measured using a thicknessmeasuring device such as a surf-analyser (e.g. TALYSURF (a contactmeasuring device which analyses the substrate surface in 2 dimensionsusing a spring loaded stylus; any change in height deflects the stylus,which is registered on a recorder, such as a chart recorder; thedifference between the base line and average height gives the printthickness)). Resistance of the pattern is determined by placing theprobe tips at the point where the conductive track meets the solderpads. The bulk resistivity (thickness-normalised) of the layer isdetermined by dividing the measured resistance for the pattern by thenumber of squares therein where the number of squares is the length ofthe conductive track divided by the width of the track. The resistivityvalue is obtained as m·/Y at a normalised thickness, herein 10 μm, andpresented herein in the units of μ·cm.

Particle Size

Particle size in the composition is measured according to ASTM D1210-79using a large Hegman type fineness of grind gauge.

Chemical Durability

A solution of 1% glacial acetic acid in deionised water is used in thistest. The glass substrate (50×100 mm) having thereon a fired conductivepattern is inserted into a plastic container half-filled with the testsolution. The container is then sealed and left to stand at ambienttemperature. The test substrates are removed after 96, 168 and 336hours, dried and then analysed by a lift test. The lift test comprisesapplication of a 0.75 inch (19.1 mm) wide masking tape (Niceday™) ontothe substrate and then removing sharply in approximately ½ second. Theresults of the lift test are given as the approximate percentage of filmarea removed by the tape.

The invention will now be described with reference to the followingexamples. It will be appreciated that the examples are not intended tobe limiting and modification of detail can be made without departingfrom the scope of the invention.

EXAMPLES

Conductive patterns were prepared using the method hereinbeforedescribed. The metal particles used were spheroid particles. The nickelparticles were sub-100 mesh, the iron particles were sub-325 mesh, thebismuth particles were sub-200 mesh and the cobalt particles were <2 μm.The silver particles were a mixture of 50% spherical silver particles(surface area of 0.80-1.40 m²g⁻¹) and 50% flake silver particles(surface area 0.60-0.90 m²g⁻¹). The glass used was Composition I inTable 1 herein. The liquid vehicle was ethyl cellulose in terpineol (ina ratio of 1 to 9) combined with the monobutyl ether of diethyleneglycol (sold as butyl Carbitol™). The substrate was a float glass(non-tempered) substrate. The fired film thickness was from 8 to 20 μm.All parts were fired through a belt furnace with a peak firingtemperature of 660° C., unless otherwise specified, with the samplesspending approximately 72 s at peak temperature. The total door-to doortransit time in the furnace was approximately 21 minutes.

The resistivity and solder adhesion of the patterns were measured as afunction of composition in accordance with the procedures describedabove and the results are shown in Table 2 below.

TABLE 2 Silver Glass Component (c) ρ W Example (% of solids) (% ofsolids) (% of solids) (μ · cm) (Kg) a: Adhesion Strength (W) andresistivity (ρ) as a function of composition (Co & Ni) Cobalt 1 75.323.82 20.86 30.90 >20 2 75.68 5.41 18.92 15.70 >20 3 63.58 18.94 17.4822.40 16.00 4 61.75 12.60 25.65 38.90 19.00 5 56.76 5.41 37.84 71.2020.00 6 53.70 8.39 37.90 119.00 15.00 7 47.68 18.94 33.38 181.00 15.00Nickel 8 75.32 3.82 20.86 17.00 >20 9 72.01 4.17 23.82 10.77 >20 1066.24 4.02 29.74 15.44 >20 11 56.66 3.78 39.57 22.08 16.00 12 47.11 3.5349.35 36.17 10.00 13 69.14 4.94 25.93 13.88 >20 14 63.28 10.49 26.2318.36 >20 15 60.61 21.72 17.68 16.78 16.00 16 57.29 16.18 26.54 24.3718.00 17 54.14 14.93 30.93 37.80 >20 18 51.45 20.48 28.07 31.36 >20 1942.60 19.31 38.10 106.40 >20 b: Adhesion Strength (W) and resistivity(ρ) as a function of composition (Fe & Bi) Iron 20 75.32 3.82 20.8610.77 >20 21 64.76 9.09 26.15 10.80 >20 22 58.60 9.17 32.23 16.20 20.0023 45.98 9.32 44.70 35.70 12.00 24 60.00 5.00 35.00 15.60 >20 25 55.499.21 35.31 18.70 >20 26 53.78 5.04 41.18 22.80 18.00 Bismuth 27 75.323.82 20.86 8.1 >20 28 50.63 5.06 44.30 11.40 14.00

The data demonstrate that the compositions described herein allow thepreparation of conductive patterns which exhibit increased resistivitywhile maintaining solder adhesion.

The resistivity/firing temperature relationship and adhesion/firingtemperature relationship of a nickel-containing pattern was measured inaccordance with the procedures described above and the results are shownin Table 3 below.

TABLE 3 Stability of resistivity to variation of firing temperatureSilver Glass Nickel FIRING TEMPERATURE (% of (% of (% of 620° C. 640° C.660° C. 680° C. solids) solids) solids) ρ W ρ W ρ W ρ W 51.45 20.4828.07 33.44 >20 30.68 >20 31.36 >20 31.09 >20 Resisitivity (ρ) measuredin μ · cm Adhesion (W) measured in Kg

The data in Table 3 demonstrate that the nickel-containing compositionsallow the preparation of conductive patterns which exhibit a lowvariation of resistivity and adhesion with variation of firingtemperature.

1-19. (canceled)
 20. A composition, useful for increasing theresistivity and adhesion of an electrically conductive patternmanufactured from such composition, comprising finely divided particlesof (a) silver; (b) one or more inorganic binders; and (c) nickel,wherein the Ag:Ni ratio is between about 1:1 and 25:1, whereincomponents (a), (b) and (c) are dispersed in a liquid vehicle,preferably an organic medium, wherein component (a) is present inamounts of about 50 to about 98% by weight of the total solids presentin the composition, and wherein component (c) is present in amounts ofabout 15 to about 45% by weight of the total solids present in thecomposition and wherein use of the composition results in resistivityincrease to at least 10 μΩcm and the adhesion increased to greater than10 kg.
 21. A composition, useful for increasing the resistivity andadhesion of an electrically conductive pattern manufactured from suchcomposition, comprising finely divided particles of (a) silver; (b) oneor more inorganic binders; and (c) iron, wherein components (a), (b) and(c) are dispersed in a liquid vehicle, wherein the liquid vehicle is anorganic medium, wherein use of the composition results in resistivityincrease to at least 10 μΩcm and the adhesion increased to greater than10 kg.
 22. A composition, useful for increasing the resistivity andadhesion of an electrically conductive pattern manufactured from suchcomposition, comprising finely divided particles of (a) silver; (b) oneor more inorganic binders; and (c) bismuth, wherein components (a), (b)and (c) are dispersed in a liquid vehicle, wherein the liquid vehicle isan organic medium, and wherein component (c) is present in amounts ofabout 15 to about 45% by weight of the total solids present in thecomposition wherein use of the composition results in resistivityincrease to at least 10 μΩcm and the adhesion increased to greater than10 kg.
 23. (canceled)
 24. (canceled)
 25. (canceled)
 26. A compositioncomprising finely divided particles of (a) silver; (b) one or moreinorganic binders; and (c) nickel, wherein the Ag:Ni ratio is betweenabout 1:1 and 25:1, wherein components (a), (b) and (c) are dispersed ina liquid vehicle, and wherein total amount of components (a), (b) and(c) is about 50 to about 95% by weight of the composition.
 27. Thecomposition of claim 26 wherein component (a) is present in amounts ofabout 50 to about 98% by weight of the total solids present in thecomposition.
 28. The composition of claim 26, wherein component (c) ispresent in amounts of about 15 to about 45% by weight of the totalsolids present in the composition.
 29. (canceled)
 30. The composition ofclaim 26 wherein the liquid vehicle is an organic medium.
 31. Thecomposition of claim 26, wherein the nickel is in the form one or moreof a metallic particle, an alloy, or a derivative substantiallyconverted to nickel under the action of heat.
 32. The composition ofclaim 26, further comprising one or more components selected from thegroup consisting of: cobalt, iron, and bismuth.
 33. The composition ofclaim 26, wherein the inorganic binder comprises a glass binder.
 34. Thecomposition of claim 26, wherein the inorganic binder comprises one ormore of a transition metal oxide, a transition metal oxide precursor, ormixtures thereof.
 35. The composition of claim 34, wherein thetransition metal oxide is selected from oxides selected from the groupconsisting of: zinc, cobalt, copper, nickel, manganese, and iron; andwherein the transition metal oxide precursor is selected from oxideprecursors selected from the group consisting of: zinc, cobalt, copper,nickel, manganese, and iron.
 36. The composition of claim 30, whereinthe organic medium comprises diethylene glycol, a monobutyl ether ofdiethylene glycol, or a mixture thereof.
 37. (canceled)